Set target platform identification variables like `APPLE` and `LINUX`
as soon as the target system is identified. This makes them available
during toolchain and binutils selection.
Fixes: #23333
DOS 16-bit and 32-bit target support files for cross-compilation by
OpenWatcom.
It is used if CMAKE_SYSTEM_NAME=DOS is defined.
If CMAKE_SYSTEM_PROCESSOR=I86 is defined then 16-bit DOS target is used,
otherwise 32-bit DOS target is used.